[erlang-questions] process per task or not ?

[Aggelos Giantsios]

Good Morning!

You can spawn a process with spawn_link or spawn_monitor depending on what
you want to use.

Links are bidirectional so if your parent process dies then the worker
processes will be affected. If you use process_flag(trap_exit, true) on
your parent process, then you will receive a message when a linked process
exits. If it crashes you get {'EXIT', From, Why} or if it exits normally
you get {'EXIT', From, normal}. If you don't use process_flag(trap_exit,
true) and let's say a worker process dies, then the links would cascade the
exit to the parent and all of the workers.

Monitors are one-way. If your monitor process crashes you get the message
{'DOWN', Ref, process, Who, Reason} and if it exits normally you get
{'DOWN', Ref, process, Who, normal}. There can be nested monitors between
processes so watch out if you do not use spawn_monitor but rather setup the
monitor later with erlang:monitor/2.

Aggelos


On Sun, Nov 17, 2013 at 6:43 AM, akonsu <akonsu@REDACTED> wrote:

> thanks everyone for your help. it is clearer now.
>
> I decided to just count my processes to make sure that I do not start more
> than some maximum number.
>
> I have one more question. Are monitors designed to be used efficiently
> with a lot of very short lived processes? Are monitors inexpensive to set
> up? Is this a normal erlang programming practice to use monitors for a lot
> of short lived processes? Alternatively, I believe I can use spawn_linked
> to start my processes, and send a message from each worker, when the worker
> finishes (since otherwise I would only get a message if a linked process
> crashes). How does this latter approach compare to using monitors?
>
> thanks for help
> konstantin
>
>
>
> 2013/11/15 akonsu <akonsu@REDACTED>
>
>> Hello,
>>
>> I am looking for an advice on how to architect my system. I am just a
>> beginner...
>>
>> I have a process that receives messages and this process needs to parse
>> each message, extract some information from the parsed message, and then
>> send this information to the subscribers.
>>
>> As I read somewhere, in a situation like this, people usually spawn a
>> worker process per message, and let this worker do the work, and then send
>> the results to the publishers.
>>
>> I have an overwhelming amount of incoming messages (these are tweets that
>> I get from a twitter stream), and I am not experienced enough to be able to
>> tell whether this worker-per-task approach is the right way to do it. I am
>> afraid that my application will be flooded with workers and eventually can
>> collapse.
>>
>> I was thinking that maybe I can use a worker process pool? A process pool
>> seems unusual in an Erlang application, does it not? Although the "Learn
>> yourself Erlang..." does have a chapter on building a pool like this.
>>
>> Or maybe rate-limit the number of incoming messages?
>>
>> I would really appreciate an advice or two.
>>
>> Thank you
>> Konstantin
>>
>>
>
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